date: use custom value parser

[cakebaker]

This PR introduces ShortcutValueParser, a custom value parser for clap that not only recognizes the exact values from a list of allowed values but also shortcuts of them (for example, h, ho, hou, are recognized as hour).

Closes #4744

[tertsdiepraam]

Super nice! We can use this in almost every util

[tertsdiepraam]

I've previously implemented something very similar as a derive macro in the argument parser I was working on (which is currently paused until I find the time to work on it again). Spinning that out of that parser and into uucore could be a nice follow-up to this PR. The derive macro allows you to write this:

#[derive(Default, Debug, PartialEq, Eq, Value)]
enum Format {
    #[value("long")]
    Long,

    #[value("single-column")]
    SingleColumn,

    #[default]
    #[value("columns", "vertical")]
    Columns,

    #[value("across", "horizontal")]
    Across,

    #[value("commas")]
    Commas,
}

The code for the macro is here: https://github.com/tertsdiepraam/uutils-args/blob/2f1a9b0ed0a328d600992e08a0950a07761a8a3b/derive/src/lib.rs#L115